The subject matter of this application is related to subject matter disclosed in my U.S. patent application Ser. No. 348,747, filed even date herewith, and assigned to the same assignee as the present application. This invention relates to the field of radiography and, more particularly, to an improved method and apparatus for producing images of X-ray patterns, also called X-ray images.
As shown in FIG. 1, conventional radiography employs a source of X-rays 10 that are directed toward the anatomical region of a patient 101 that is to be examined. The number and energy of the X-rays employed are together referred to as the X-ray exposure. Part of this X-ray exposure is absorbed or scattered by the patient, and part passes through the patient and strikes the X-ray detector. Since the number of X-rays that pass through different regions of the patient differ due to differences in the nature and density of the constituent organs within the patient, the X-rays that are detected form a pattern or "shadow image" indicative of the relative densities of the tissues within the patient. In conventional radiography, this shadow image is recorded using a screen/film combination as the detector.
The typical screen/film detector works in the following way. One or two fluorescent screens, such as 121 and 122 in FIG. 1, are placed in contact with a piece of light-sensitive film, 125. The film 125 is relatively insensitive to X-ray exposure, while the screen(s) 121, 122 are highly sensitive to X-ray exposure. The combination of screen(s) and film are typically placed in a light-tight cassette (not shown) that is composed of low density material that produces little interaction with the x-rays. The X-rays that pass through the patient are partially absorbed by the first screen, which typically contains high density, high atomic number materials, such as tungsten, or rare earth elements such as gadolinium or lanthanum. Part of the X-ray energy that is absorbed by the first screen is converted into light (fluorescence), which then exposes the film. If a second screen is used, additional fluorescent light is produced in response to the further absorption of X-rays that passed through the first screen and impinged on the second screen; thereby increasing the sensitivity of the detector system.
After the X-ray exposure has been completed, the film is removed from the cassette and developed, typically using standard photographic film development techniques. The developed film looks like a film "negative" and is called a "radiograph." The developed radiograph contains a pattern of film darkening representative of the distribution of tissue densities within the patient being examined.
For any screen/film system, there exists a quantitative relationship between the amount of X-ray exposure absorbed by the screen(s) and the amount of film darkening produced. This relationship is represented in FIG. 2, the solid-line curve of which is a plot of the amount of film darkening (optical density, O.D.) produced versus the logarithm of the amount of X-ray exposure detected, LOG(E). This curve is generally referred to as either the "characteristic" or "H and D" curve. In the example of FIG. 2, the curve shown is the characteristic curve for a common X-ray film, Kodak XL-1.
The shape of the characteristic curve strongly affects the final appearance of the radiograph. For example, the steeper the curve, the greater incremental film darkening (contrast) is produced in response to an incremental increase in X-ray exposure. The slope (or gamma) of the characteristic curve is also plotted (dashed-line curve) as a function of LOG(E) in FIG. 2.
In general, it is desirable to record an image using an amount of X-ray exposure that will place the recorded film darkening near the region of steepest slope (highest gamma). Since the range of X-ray exposures recorded on a single film is great (often a factor of 50 to 100), however, one cannot have all regions of the film recorded near the region of highest slope, unless a film with relatively lower overall slope (so-called "high latitude" film) is used. The disadvantage to this approach, however, is that the radiograph produced tends to be too low in overall contrast. For this reason, films are used whose maximum slopes are in the range between about 2.5 and 3.0, and X-ray exposures are employed so that most, but not all, of the film darkening occurs near the region of highest slope. As a result, under-exposed and over-exposed regions are recorded with inferior contrast as compared to the regions recorded near highest slope. These two inferior regions are referred to as the "toe" and "shoulder" of the characteristic curve.
Another feature of radiographic films produced using standard techniques is that a very wide range of light is transmitted through a radiograph when viewed. If, for example, the range of optical densities recorded on a film is 2.0 O.D. units, a factor of 100 (10.sup.2) in light transmission results. This means that a factor of 100 in light levels must be simultaneously viewed by the radiologist when examining the film to make a diagnosis. This range of light intensities exceeds what can be viewed comfortably by the human eye. Regions of high transmission (low optical density) contribute glare, in much the same way as headlights of approaching cars produce glare when driving at night. This glare impairs the radiologist's ability to visualize subtle detail in the darker regions of a radiograph. It is among the objects of the present invention to provide solution to the above-described problems and limitations of prior art techniques.